The invention relates to a colour display tube containing
a) an evacuated envelope consisting of a neck, a cone and a display window, PA1 b) in the neck an electron gun for generating one central and two outer electron beams whose axes are coplanar, the electron gun comprising a first and a second electrode system, which in operation together form a main lens, and means for applying a focusing voltage and a high voltage to the first and the second electrode system respectively, PA1 c) in the electron gun an astigmatic element for astigmatically influencing the electron beams, and PA1 d) a deflection system for generating deflection fields for deflecting the electron beams.
A colour display tube of the type described in the first paragraph is known from European Patent Application EP-A-0231964.
In EP-A-0231964 the colour display tube contains a deflection system which during operation generates horizontal and vertical magnetic deflection fields, such that the three electron beams generated by the electron gun and focussed by the main lens on a display screen provided internally on the display window converge over the entire display screen. This leads to a vertical over-focussing of the electron beams on the display screen. This vertical overfocussing can be compensated in part by means of a static astigmatic element; however, in the case of applications which impose ever higher requirements on the definition such as, for example, high resolution colour display tubes, this is sometimes insufficient. In EP-A-0231964 a construction of an electron gun is described which enables a substantially complete correction of the vertical overfocussing by dynamically varying the strength of the astigmatic element with the strength of the deflection fields.
However, on deflection the horizontal spot size increases by a certain spot enlargement factor also, which factor amounts to more than two in the case of 110.degree. colour display tubes. The spot does remain focussed or substantially focussed in the horizontal direction over the entire display screen. In the known construction this horizontal spot enlargement factor is reduced to a very small extent only or not at all. Due to the ever higher requirements imposed on the definition of the display, in particular in the case of high resolution colour display tubes or in the use of colour display tubes for high definition television, it is also important to reduce the, horizontal spot enlargement factor.